1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) in which a MTJ (Magnetic Tunnel Junction) element for storing “1”, “0”-information by a TMR effect is used to constitute a memory cell.
2. Description of the Related Art
In recent years, a large number of memories in which information is stored by a new principle have been proposed. Among the memories, there is a memory which uses a tunneling magneto resistive (hereinafter referred to as TMR) effect proposed by Roy Scheuerlein et al. (refer to ISSCC2000 Technical Digest p. 128 “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”).
In a magnetic random access memory, a MTJ element stores “1”, “0”-information. As shown in FIG. 1, the MTJ element includes a structure in which an insulating layer (tunneling barrier) is held by two magnetic layers (ferromagnetic layers). The information to be stored in the MTJ element is judged by judging whether or not directions of spin of two magnetic layers are parallel or anti-parallel.
Here, as shown in FIG. 2, “parallel” means that the directions of spin of two magnetic layers (direction of magnetization) are the same, and “anti-parallel” means that the directions of spin of two magnetic layers are opposite to each other (the directions of arrows indicate the directions of spin).
It is to be noted that an anti-ferromagnetic layer is usually disposed in one of two magnetic layers. The anti-ferromagnetic layer is a member for fixing the spin direction of one magnetic layer and changing only the spin direction of the other magnetic layer to easily rewrite the information.
The magnetic layer whose spin direction is fixed is referred to as a fixed or pinned layer. Moreover, the magnetic layer whose spin direction can freely be changed in accordance with write data is referred to as a free or storage layer.
As shown in FIG. 2, when the spin directions of two magnetic layers are parallel to each other, tunnel resistance of the insulating layer (tunneling barrier) held between two magnetic layers is lowest. This state is a “1”-state. Moreover, when the spin directions of two magnetic layers are anti-parallel, the tunnel resistance of the insulating layer (tunneling barrier) held between two magnetic layers is highest. This state is a “0”-state.
A write operation principle with respect to the MTJ element will next briefly be described with reference to FIG. 3.
The MTJ element is disposed in an intersection of a write word line and data selection line (read/write bit line) which intersect with each other. Moreover, the write is achieved by passing a current into the write word line and data selection line, and using a magnetic field formed by the current flowing through both wires to set the spin direction of the MTJ element to be parallel or anti-parallel.
For example, when a magnetization easy axis of the MTJ element is an X-direction, the write word line extends in the X-direction, and the data selection line extends in a Y-direction crossing at right angles to the X-direction, a current directed in one direction is passed through the write word line at a write time, and a current directed in one or the other direction is passed through the data selection line in accordance with the write data.
When the current directed in one direction is passed through the data selection line, the spin direction of the MTJ element becomes parallel (“1”-state). On the other hand, when the current directed in the other direction is passed through the data selection line, the spin direction of the MTJ element becomes anti-parallel (“0”-state).
A mechanism in which the spin direction of the MTJ element changes is as follows.
When a magnetic field Hx is applied in a long-side (easy-axis) direction of the MTJ element as shown by a TMR curve of FIG. 4, the resistance value of the MTJ element changes, for example, by about 17%. This change ratio, that is, a ratio of the resistance values before and after the change is referred to as an MR ratio.
It is to be noted that the MR ratio changes by properties of the magnetic layer. At present, the MTJ element whose MR ratio is about 50% is obtained.
A synthetic magnetic field of the magnetic field Hx of an easy-axis direction and magnetic field Hy of a hard-axis direction is applied to the MTJ element. As shown by a solid line of FIG. 5, the size of the magnetic field Hx of the easy-axis direction necessary for changing the resistance value of the MTJ element also changes by the size of the magnetic field Hy of the hard-axis direction. This phenomenon can be used to write data only into the MTJ element which exists in the intersection of the selected write word line and data selection line among memory cells arranged in an array form.
This state will further be described with reference to Astroid curve of FIG. 5.
The Astroid curve of the MTJ element is shown, for example, by a solid line of FIG. 5. That is, when the size of the synthetic magnetic field of the magnetic field Hx of the easy-axis direction and the magnetic field Hy of the hard-axis direction is outside the Astroid curve (solid line) (e.g., in positions of black circles), the spin direction of the magnetic layer can be reversed.
Conversely, when the size of the synthetic magnetic field of the magnetic field Hx of the easy-axis direction and the magnetic field Hy of the hard-axis direction is inside the Astroid curve (solid line) (e.g., in positions of white circles), the spin direction of the magnetic layer cannot be reversed.
Therefore, when the sizes of the magnetic field Hx of the easy-axis direction and the magnetic field Hy of the hard-axis direction are changed, and the position of the size of the synthetic magnetic field in an Hx-Hy plane is changed, the write of the data with respect to the MTJ element can be controlled.
A read operation can easily be performed by passing a current through the selected MTJ element, and detecting the resistance value of the MTJ element.
For example, a switch element is connected in series to the MTJ element, and only the switch element connected to a selected read word line is turned on to form a current path. As a result, since the current flows only through the selected MTJ element, the data of the MTJ element can be read out.
In the magnetic random access memory, as described above, the data write is performed by passing the write current through the write word line and data selection line (read/write bit line) and allowing a synthetic magnetic field Hx+Hy generated thereby to act on the MTJ element.
Therefore, to efficiently perform the data write, it is important to efficiently apply the synthetic magnetic field Hx+Hy to the MTJ element. When the synthetic magnetic field Hx+Hy is efficiently applied to the MTJ element, reliability of the write operation is enhanced, further a write current is reduced, and low power consumption can be realized.
However, an effective device structure for allowing the synthetic magnetic field Hx+Hy generated by the write currents flowing through the write word line and data selection line to efficiently act on the MTJ element has not been sufficiently studied. That is, for the device structure, it naturally needs to be studied whether the synthetic magnetic field Hx+Hy is actually efficiently applied to the MTJ element. Furthermore, in a manufacturing process aspect, it needs to be studied whether or not the structure can easily be manufactured.
In recent years, as a technique of efficiently applying the magnetic fields Hx, Hy to the MTJ element, the device structure has been studied in which a yoke material having a function of suppressing spread of the magnetic field is disposed around a write line (refer to U.S. Pat. No. 6,174,737).
The yoke material has high permeability, and magnetic flux has a property of being concentrated on a material which has the high permeability. Therefore, when the yoke material is used as a traction material of a magnetic force line, the magnetic fields Hx, Hy generated by the write current flowing through the write line can efficiently be concentrated on the MTJ element at a write operation time.
The yoke material has a function of suppressing the spread of the magnetic field as described above. This is based on a prerequisite that film thickness and magnetic domain of the yoke material are accurately controlled. That is, when dispersion is generated in the film thickness of the yoke material arranged around the write line, and the magnetic domain is not orderly aligned, an effect of the yoke material in bunching a magnetic force line is reduced, and it becomes impossible to efficiently apply the magnetic fields Hx, Hy to the MTJ element.